This Phase II, randomized, partially placebo controlled, 4-arm trial compares amprenavir (APV) in a single-protease inhibitor (PI) regimen to APV in combination with saquinavir soft gel caps (SQVsgc), indinavir(IDV), or nelfinavir (NFV) in HIV-infected subjects currently failing indinavir, ritonavir, saquinavir, or nelfinavir as part of a single or dual PI regimen, as evidenced by plasma HIV-1 RNA concentration > 1,000 copies/ml. Virologic response, safety and tolerance will be evaluated across the study arms. Plasma HIV-1 RNA concentrations, CD4 counts, and drug resistance profiles will be determined. Pharmacokinetics will be obtained.